CATHODIC PROTECTION – LUXURY OR NECESSITY?


The lifeblood of modern civilization – crude oil and its derivatives come from natural sources. This was once luxuriant vegetation which grew in a pristine environment and which has transformed with time and under immense pressure into liquid gold – a commodity which has changed the face of humanity. Unfortunately, after being brought to the surface of the earth it becomes not just a blessing, but also a curse, not only a source of life-giving energy, but also a source of destruction. It is not only a remedy, but also a poison.

There is nothing original in these words. Everyone today knows this. The carcinogenic properties of petroleum derivatives are well known, as is the fire hazard. Awareness of dangers to the natural environment has appeared recently – merely twenty, maybe thirty years ago. Today many people know that one litre of hydrocarbons makes one million litres of drinking water useless. Protection against such dangers is finding its way into the provisions of standards and regulations.

From the moment it leaves the ground, crude oil is harnessed into steel pipes and tanks which not only make its transport possible, but also separate it from the surroundings. Before it ends up in a gas tank of a car and undergoes combustion in the engine, for example, it will flow through hundreds of kilometres of such pipes, visit a few dozen storage tanks and the interior of a petrochemical installation. Are we aware that the only thing which separates us from this dangerous substance is the steel wall of a pipe, tank or reactor which is barely several millimetres thick?

Just as crude oil and natural gas have become fundamental sources of energy, our civilization uses steel as the basic construction material. Together with concrete they form the image of the modern human world. Where they are used depends only on the imaginations and talents of designers and builders. Ordinary carbon steel is popular mainly thanks to its low price as compared to its benefits, in particular its mechanical endurance. Unfortunately, this excellent material has two extremely annoying weaknesses: steel naturally deteriorates in the surrounding environment, i.e. it corrodes, and it loses its mechanical properties at high temperatures, such as in a fire. Using steel for technological purposes is invariably connected with protecting it against these destructive factors. And, of course, nothing is free. Such protection costs money.

So, if crude oil and its products, all dangerous to the natural environment, are separated from it only by a thin layer of steel, the durability and effectiveness of this barrier will depend on the resistance of the material against natural deterioration. In other words, the tightness of the “wall” depends directly on the corrosion of the steel, and in practice on how the steel is protected against corrosion. Crude oil itself is a corrosive substance during extraction and during transport and storage, as it’s mixed with highly saline and very aggressive formation water. On the other side of the wall is the surrounding atmosphere, water or soil – environments in which steel cannot be used without appropriate corrosion protection. This seemingly simple relation between corrosion protection of steel, the expense of using it and the effectiveness of protection of the natural environment (with respect to the transport and storage of this type of hazardous substances) is not known as well as it should be, especially in ecological circles. What’s worse, there is a search for environmental protection methods whose aim is not to improve the quality of corrosion protection of the steel walls (a direct cause of environmental pollution), but to reduce the possibility of hazardous substance leaks from damaged walls (due to loss of leak tightness). Should the objective not be to eliminate the cause, rather than the effect? Is the underlying reason for such action a lack of faith in the possibility of effective protection against the corrosion of the wall? Is this admission of defeat in the fight against the corrosion of steel?

Changes taking place in the environment around us clearly show that it is possible to win the war against corrosion. The advancement of science and accumulated knowledge and experience have made themselves obvious. Rusty cars are gone from the streets, periods between repairs of ships, machines and installations have been extended and new technologies and anti-corrosion materials have appeared. Various corrosion protection technologies are used with increasing consideration and technical justification. The related costs are incurred with greater courage. Fighting corrosion is increasingly less associated with a scraper, a can of paint and paintbrush.

Corrosion protection is generally associated with protective coatings. This is understandable, as in normal atmospheric conditions this is a standard technique of protecting steel against corrosion. Coatings do not only perform a decorative function, but they also counteract the reduction of thickness of steel elements and thus the reduction of its mechanical durability. Unfortunately coatings are never completely tight. They always have minor defects which appear at the time they are made or later, due to their aging. Therefore, the likelihood of a certain area of the metal being in direct contact with the environment is quite large. Does minor damage or even a hole in a steel construction element of a bridge make it unfit for use? Of course not – this kind of damage does not have a significant impact on the mechanical durability of the structure – a property essential for the bridge to function. A wholly different conclusion can be drawn when one considers such a hole in a pipeline or fuel storage tank. Even the smallest damage makes the whole facility unfit for operation, because its fundamental property should be leak tightness. Isn’t this obvious from an environmental protection viewpoint?

Loss of leak tightness of a corrosion protection coating on a steel structure situated in open air is first apparent as a rusty spot, then a patch, a seep and finally a perforation. The defect is easily seen with the naked eye. The products of corrosion are easily removed and the protective coating regenerated. However the situation is quite different under water or underground. Not only because the effects of corrosion processes cannot be seen easily. The difference is significant because as opposed to air, water and soil conduct electric current and even though the electro-chemical corrosion mechanism is identical, the process takes place much faster. If electric current may travel through defects in the protection layer into water or soil, metal damage will always occur in this spot due to an accelerated oxidation process regardless of the type or quality of the protection layer. The process is autocatalytic and accelerates over time. Oxidation leads to acidification in the spot where the current travels through the damaged spot. This reduces electric resistance and increases current intensity, which in turn accelerates the oxidation process and so on, until a perforation appears. The driving force of this process is the current flowing between various elements of the same structure due to potential differences between these elements or due to current flow from other sources. It’s worth noting that a 1 ampere current will destroy about 10 kg of iron per year.

Can this destructive steel corrosion process under water and underground be stopped if it occurs regardless of the type of the protection layer used? It turns out the answer is yes – also by using electric current in a special method called cathodic protection. When using it, electric current is forced to flow in the reverse direction to the direction described above, thus arresting the corrosion process completely. The conclusion is, that under water and underground, the only way to use steel structures effectively is to employ cathodic protection against their corrosion. The biggest advantage of this method is that when used in combination with protection layers, it works only in places where the protection layer is damaged, which are exactly the spots where the risk of corrosion and ensuing perforation is the greatest, leading to the substance leaking out into the environment. The compatible action of these two methods guarantees reliable operation of structures under water and underground. Unfortunately the application of this technology is difficult and it should be used only under the supervision of experienced experts.

This is not groundbreaking information. It can be found in any textbook on corrosion and protection of metals. The problem is that there are no such books on the market. There is also a shortage of experts and education of engineers in this field is exceedingly meager. After the death of Professor Juchniewicz, the Electro-Chemical Protection Against Corrosion Team which he headed at the Gdańsk University of Technology ceased to exist. The co-founders of the team modestly continue scientific and technical activity in their own company. The mission of Specjalistyczne Przedsiębiorstwo Zabezpieczeń Przeciwkorozyjnych CORRPOL Sp. z o.o. in Gdańsk is to disseminate knowledge and implement cathodic protection technology in Polish industry and economy on an extensive scale.


Wojciech Sokólski, 2004